The switching power supply has advantages of small volume, high efficiency, and large current, and is widely applied in a mobile phone charger, an adapter and so on. At present, the switching power supply realizes communication between the primary side and the secondary side of the transformer through two kinds of messaging modes. One messaging mode is shown in FIG. 1. The system comprises a primary controller, a transformer, a power switch, a communication chip, an optical coupler, a load, a CS sampling loop connected with the primary controller, a DC output stage connected with the secondary side of the transformer, a VDD power supplying loop connected with an auxiliary coil of the transformer and a feedback loop FB. The system realizes communication between the primary side and the secondary side of the transformer by the following working principle: when the communication chip receives a communication enabling signal from the load, the communication chip drives the optical coupler to transmit the signal to the pin DDC (Digital Date Communication) of the primary controller. The primary controller records the signal at the pin DDC, and actuates corresponding actions according to the signal, thereby realizing communication. Although the communication between the secondary side and the primary side of the transformer can be realized in this way, the optical coupler or other devices for communicating should be provided additionally, thereby increasing the cost of the system. Additionally, an extra port for the primary controller of the primary feedback should be configured to receive the communication signal.
As shown in FIG. 2, another implementation of the system comprises a primary controller, a transformer, a power switch, a secondary controller, a load, a CS sampling loop connected with the primary controller, a DC output stage connected with the secondary side of the transformer, a VDD power supplying loop connected with an auxiliary coil of the transformer, and a feedback loop FB. The system realizes communication between the primary side and the secondary side of the transformer by the following working principle: when the secondary controller detects the communication signal output from the load and compiles a switching pulse signal VG in the dead time according to the content of the communication signal. The switching pulse signal causes the system to oscillate, and the oscillation is fed back to the pin FB of the primary controller through the secondary winding and the auxiliary winding. The primary controller monitors the oscillation signal at the pin FB and actuates corresponding actions according to the content of the signal. FIG. 3 shows waveforms of the communication in the dead time. The communication between the primary side and the secondary side of the transformer can be realized in this way, and no additional devices are required, but the communication is implemented in the dead time, which requires that the primary controller works in DCM. What's more, in the dead time, the secondary diode of the switch is apt to cause oscillation between the inductor of the transformer and the primary parasitic capacitance, which is unfavorable for the primary controller to identify the communication signal.